Fix cast_motion sometimes failing

- Fixes Godot physics failing when the cast Shape is inside of, or
already colliding with another Shape.
- Fixes Bullet physics failing when there is no motion.
- Ensures Godot and Bullet physics behave the same.
- Updates the documentation to exclude the caveats for the failures and
differences.

servers/physics/space_sw.cpp

@@ -265,11 +265,11 @@ bool PhysicsDirectSpaceStateSW::cast_motion(const RID &p_shape, const Transform
 			continue;
 		}
 
-		//test initial overlap
+		//test initial overlap, ignore objects it's inside of.
 		sep_axis = p_motion.normalized();
 
 		if (!CollisionSolverSW::solve_distance(shape, p_xform, col_obj->get_shape(shape_idx), col_obj_xform, point_A, point_B, aabb, &sep_axis)) {
-			return false;
+			continue;
 		}
 
 		//just do kinematic solving